The conventional excess-air incineration process attempts to achieve complete combustion of waste materials and combustion gases in the main combustion chamber. To achieve this objective, a large volume of air is introduced into the mass of the waste material, and secondary air is introduced into the upper portion of the combustion chamber in order to burn the combustible gases generated in the initial stage of combustion. The introduction of large volumes of air in the combustion chamber creates high turbulence which entrains ash and other particulate matter in the stack gases. The entrained material, unless removed from the stack gases by further processing can cause serious air pollution problems.
In a controlled air type of incineration process, lesser volumes of air are utilized and the amount of air supplied to the combustion chamber is controlled or varied in accordance with the rate of combustion. The controlled air system attempts to achieve only partial combustion of the waste material at high temperatures, resulting in the pyrolysis of organic matter to combustible gases which are completely combusted in a secondary thermal reaction zone.
A starved air combustion system also operates on pyrolytic principles and differs from a controlled air system in that air is introduced at a constant rate only into the lower end of the combustion chamber and no air is introduced into the upper portion of the combustion chamber. The combustible gases are completely combusted by thermal reaction in the stack by introducing either forced air, or air by natural aspiration, into the stack.
The use of a starved air system has advantages over a conventional incineration process in that there is less turbulence in the main combustion chamber, so that the quantity of non-combustibles in the stack gases is minimized. The starved system also provides economies in fuel and air requirements over a conventional incineration process.
When burning certain highly compacted materials such as computer program cards, telephone books, or material that become amorphous when heated, is is difficult to introduce the air into the mass of the highly compacted waste. With the conventional excess-air incineration process, movable grates are frequently employed which act to agitate the waste material to obtain better air-waste contact. However, movable grates are not normally used with a starved air system because of the high temperatures in the bed, and the grates without cooling provided by the relatively high underfire air flow, such as required by the conventional incinerator, would have a limited service life at these extreme temperatures.